The apparatus and method of the present invention relate to lens components for multispectral optical systems, and, more particularly, to, for example, a zinc selenide/zinc sulfide (ZnSe/ZnS) doublet structure used in infrared common aperture optical systems.
In a common aperture infrared device, a laser is used to designate and find the range of targets in the scene, and a Forward Looking Infrared device ("FLIR") views the scene. In these devices, the laser and the FLIR share a common optical train in order to insure that the FLIR views the scene illuminated by the laser. Thus, the laser source will pass through one or more of the FLIR elements, as well as the shared device window.
Typically, these FLIRs view in the 8-12 micron waveband, while the illumination source is typically a Neodimium YAG laser operating at 1.06 microns. One obvious requirement of the shared lenses is that they transmit in both spectral ranges, 8-12 microns and 1.06 microns. In addition, the lens material must have stable mechanical properties, and the desired optical properties, such as index of refraction and dispersion.
Often such systems will use a ZnSe/ZnS doublet as the shared optical element, for example, as a FLIR telescope objective which is then shared with the laser designator. In the past, these doublets have been constructed by individually fabricating a positive ZnSe element and a negative ZnS element. A combination of these materials is required to compensate for the high relative dispersions of each respective material.
Such ZnSe/ZnS doublet design has several disadvantages. First, unlike many infrared telescope objectives, it includes two lenses rather than one. Since each element in the doublet must be thick enough to be individually structurally rigid, a doublet weighs more than a functionally similar singlet. This extra weight may be a critical problem in many airborne systems. Second, the transmittance of the lens is diminished because the ZnS element, which has relatively high absorption in the 8-12 micron waveband, is necessarily thick in order to provide structural rigidity to the lens.
It is, accordingly, a primary object of the present invention to provide an improved sandwich lens which is lighter and has higher transmittance, and has lower material cost than a traditional ZnSe/ZnS doublet.